CPM tutor

a practical guide to construction scheduling

noteable quote

"Facts are ventriloquists’ dummies. Sitting on a wise man’s knee they may be made to utter words of wisdom; elsewhere, they say nothing, or talk nonsense, or indulge in sheer diabolism." - Aldous Huxley (1944)

dummies

Since any real Arrow Diagram can be expected to include
logic and numbering dummies, let’s do an example now
with an Arrow Diagram that has logic dummies. The example
project is shown below as five work activities and two
dummy activities.

Given the standard project start date of day one (1),
the Early Event Times at node-10 and node-15 are easy to
calculate. Now we have dummies between nodes 10-20 and
15-20 to calculate. The first thing to note is that dummy
activities have, by definition, zero duration. Given that
we can calculate the Early Event Time for node-20 just like
any other activity with multiple incoming activities. The
selection of the Early Event Time for a given node is the
latest value at the end of any of the incoming activities.
Since the Early Event Time from node-15 plus the duration
of the dummy activity from node-15 to node-20, i.e. zero (0),
is later than the Early Event Time provided from the dummy
at node-10 to node-20, the Early Even Time at node-20 is 7.

Once we know the Early Event Time of node-20 we can calculate the
Early Event Time at node-25. The latest of each of the incoming Early
Event Times is 15. This value comes from the Early Event Time from
node-20 plus the activity duration of Activity D.

The longest two activities in the network are Activity B and Activity
D. These activities govern the length of the whole project. It is interesting
to note that the longest path through the network from node-5, to node-15,
to node-20, to node-25 contains a dummy activity with zero duration!